1. Field of the Invention
The present invention relates to a method for manufacturing a metal carrier of a catalytic converter to be used for an exhaust gas purifying device of internal combustion engine.
2. Related Art Statement
Conventionally, it has been practiced to manufacture a metal carrier of a catalytic converter to be used for an exhaust gas purifying device of internal combustion engine, by alternatively overlapping a pair of flat metal foil 2 and wavy metal foil 3 such as made of heat-resistant stainless steel, spirally winding them in this state, and joining respective top portions of waves of the wavy metal foil 3 to a corresponding surface of the flat metal foil 2 such as by brazing, as shown by a reference numeral 1 in FIG. 1.
The thus manufactured catalyst carrier 1 made of metal has a number of exhaust gas passages defined by the flat metal foil 2 and wavy metal foil 3. As shown in FIG. 2, wash coat liquid 4 is applied onto the surfaces of the exhaust gas passages with dipping, and then dried. Thereafter, catalyst is carried on the surfaces of the wash coat liquid 4 to thereby manufacture an exhaust gas purifying catalyst.
Once the exhaust gas gets into the exhaust gas passages of the exhaust gas purifying catalyst, reaction target substance within exhaust gas moves to a surface of the catalyst due to diffusion so that a predetermined chemical reaction progresses. As a result, generated substance moves from the catalyst into the exhaust gas, and is then discharged into the atmosphere.
Thus, exhaust gas purifying rate or speed is limited by: a transfer rate of the reaction target substance onto a catalyst surface; a chemical reaction rate at the catalyst surface; and a transfer rate of a generated substance from the catalyst surface. If the exhaust gas purifying rate is fast, the exhaust gas purifying catalyst may have a short length, and if the exhaust gas purifying late is slow, it is necessary to provide an exhaust gas purifying catalyst having a length sufficient for purifying a harmful substance within exhaust gas.
(1) It is a matter of course that an exhaust gas purifying catalyst preferably to have a higher reaction efficiency and having a shorter axial length is preferable.
On the other hand, in addition to the aforementioned demand, exhaust gas purifying catalyst is preferable to satisfy the following conditions.
(2) It is said that, in an automobile in which an exhaust gas purifying catalyst is used, a ratio of harmful exhaust substance to be discharged just after starting of engine, in relation to an entire quantity of harmful exhaust substance, is 50% or more. Therefore, it is extremely important that a temperature rising rate of exhaust gas purifying catalyst just after starting of engine is high so as to contribute to removal of harmful exhaust substance.
By the way, commonly used platinum based catalyst normally functions at a temperature of 350xc2x0 C. or higher. It is accordingly preferable that an exhaust gas purifying catalyst reaches this activating temperature as soon as possible after starting of engine.
Meanwhile, as previously described in relation to FIG. 1, in a conventional exhaust gas purifying catalyst, catalyst carrier 1 has a structure having overlapped flat metal foil 2 and wavy metal foil 3. Therefore, as shown by xcex1 in FIG. 2, it is impossible to avoid occurrence of surfaces intersecting with each other at an acute angle interiorly of the exhaust gas passage.
It is also impossible to avoid that an unnecessarily large amount of wash coat liquid 4 is coated between the surfaces intersecting with each other at an acute angle (xcex1) within the exhaust gas passage, since the wash coat liquid 4 is coated onto an inner surface of the exhaust gas passage by a dipping method as described above so that the wash coat liquid 4 concentrates to the aforementioned area due to a surface tension.
As such, in the conventional catalyst carrier 1 for an exhaust gas purifying catalyst, in addition to a problem of cost increase due to adhesion of excessive amount of wash coat liquid, there have been caused such problems that:
Deterioration of catalyst reaction efficiency due to reduction of catalyst carrying surface area inevitably leads to a long and large exhaust gas purifying catalyst, so that the aforementioned demand (1) is not satisfied; and
Increase of heat capacity due to adhesion of excessive amount of wash coat liquid prolongs a time required for temperature rise up to the activating temperature of exhaust gas purifying catalyst after starting of engine, so that the aforementioned demand (2) is not fully satisfied.
Further, it has been also confirmed that the following problems are caused in the conventional catalyst carrier 1 for exhaust gas purifying catalyst.
Namely, flow rate of exhaust gas entering the catalyst carrier 1 is not uniform. Generally, high speed exhaust gas flows such as from an exhaust pipe having a diameter of about 60 mm or less into the catalyst carrier 1 having a diameter of approximately 100 mm, so that the flow rate is high at a center portion and low at a peripheral portion of the catalyst carrier 1.
At the center portion of catalyst carrier 1 at which the flow rate is high, temperature of wall surface rises within a short period of time just after starting of engine. However, at the peripheral portion of catalyst carrier 1 at which the flow rate is low, wall surface does not reach an activating temperature unless a considerable period of time has passed after starting of engine, resulting in that the temperature elevation of wall surface just after starting of engine is delayed during which unpurified harmful substance continues to flow out.
To solve the aforementioned problems, as described in Japanese Patent Application Opened No. 309277/93, there has been proposed a countermeasure to penetratingly form a number of holes in a flat metal foil and a wavy metal foil so as to diffuse the exhaust gas in a radial direction within the carrier.
However, in case of penetratingly forming a number of holes newly in the flat metal foil and wavy metal foil, it is required to provide another process for forming the holes, leading to increase of cost. Among other things, it has been confirmed that the degree of performance improvement of the peripheral portion of carrier is low relative to the increased cost, thus this is not practical.
It was pointed out in the above that the heat capacity of the conventional catalyst carrier 1 is one of the reasons which make the delay of the temperature rise of catalyst after starting of engine. In addition, a low heat transfer rate from exhaust gas to wall surface of carrier is the other reason which makes the delay of the temperature rise of catalyst.
Considering here a heat transfer rate of exhaust gas to a wall surface of carrier, it is apparent that: the shorter the distance between the exhaust gas and the catalyst surface, the shorter the period of time required that all of reactants reach the catalyst surface and are substituted by reaction products by transference of reactants within the exhaust gas passage.
To shorten the distance between exhaust gas and catalyst surface, it is sufficient to reduce a cross sectional area of the exhaust gas passage insofar as the cross sectional shapes are identical. Further, concerning a cross sectional shape of exhaust gas passage, the object of interest is achieved by flattening the cross sectional shape to thereby shorten a distance between opposing wall surfaces of exhaust gas passage.
Concerning the latter cross sectional shape of exhaust gas passage, in xe2x80x9cAnalytical Investigation of the Performance of Catalytic Monoliths of Varying Channel Geometries Based on Mass Transfer Controlling Conditionsxe2x80x9d, Society of Automotive Engineers, Automotive Engineers Congress, Feb. 25, 1974, there has been presented a calculation result obtained by: successively changing a cross sectional shape of exhaust gas passage such as into triangular, circular, square, rectangular shape; calculating a reaction rate within the exhaust gas passage; and thereby obtaining such as a length of exhaust gas purifying catalyst required for completing the reaction, and pressure loss due to passing through carrier.
According thereto, it has been clarified that the most superior mass transfer rate is presented by a rectangular cross sectional shape of exhaust gas passage having an aspect ratio of about 4 or more.
Heat transfer from exhaust gas to a catalyst wall surface is performed through collision of exhaust gas molecule with the catalyst wall surface. As such, there can be generally found a positive correlation between a mass transfer rate from exhaust gas to catalyst wall surface and a heat transfer rate. Thus, by selecting a cross sectional shape the mass transfer rate is high to thereby promote a catalyst reaction, the heat transfer rate is necessarily improved. By rendering the cross sectional shape of exhaust gas passage to be a rectangle having an aspect ratio of about 4 or more as described above, there can be expected the fastest temperature rise of catalyst, and there can be effectively promoted a temperature rise of catalyst after starting of engine.
Contrary, as clearly shown in FIG. 2, in the conventional metal carrier, the exhaust gas passage has nearly a triangular cross sectional shape which presents an inferior heat transfer rate from exhaust gas to the wall surface of carrier, also resulting in delay of temperature rise of catalyst.
It is therefore an object of the present invention, in stead of manufacturing a metal carrier for an exhaust gas purifying catalyst by overlapping a flat metal foil and wavy metal foil in view of the aforementioned actual situation, to manufacture a catalyst carrier basically by laminating flat metal foils only so as to form an exhaust gas passage by separating flat metal foils with projections provided in flat metal foils by a predetermined distance, and so as to render the flat metal foils to be hole opened such that exhaust gas is allowed to flow also in a radial direction, to thereby propose a manufacturing method of a metal carrier usable for an exhaust gas purifying catalyst adapted to fully solve the aforementioned problems.
It is another object of the present invention to propose a manufacturing method of a metal carrier usable for an exhaust gas purifying catalyst in which joining strength between flat metal foils is increased.
It is another object of the present invention to propose a manufacturing method of a metal carrier usable for an exhaust gas purifying catalyst in which strength at an exhaust gas inflow side portion is increased.
It is another object of the present invention to propose a manufacturing method of a metal carrier usable for an exhaust gas purifying catalyst in which the projections and holes to be provided at the flat metal foil are simultaneously and readily formed without accompanying weight increase.
It is another object of the present invention to propose a manufacturing method of a metal carrier usable for an exhaust gas purifying catalyst in which the formation of the projections and holes according to the fourth invention can be performed such that wrinkles are not caused in the flat metal foils.
It is another object of the present invention to propose a manufacturing method of a metal carrier usable for an exhaust gas purifying catalyst in which the projections do not give a resistance so large as to be problematic to an exhaust gas flow, nor cause an inconvenience for a spiral winding operation of the flat metal foils.
It is another object of the present invention to propose a manufacturing method of a metal carrier usable for an exhaust gas purifying catalyst in which the projections do not cause a spring back when the projections are formed by stamping.
It is another object of the present invention to propose a manufacturing method of a metal carrier usable for an exhaust gas purifying catalyst in which shearing stresses acting on the punch for stamping the flat metal foil are compensated with each other, when the projections are formed by stamping.
It is another object of the present invention to propose a manufacturing method of a metal carrier usable for an exhaust gas purifying catalyst in which there is avoided such a situation that the projections align with the holes when the flat metal foils are overlapped with each other so that the projections fail to maintain the spacing between the flat metal foils in a predetermined manner.
First of all, for the aforementioned objects, the present invention provides a method for manufacturing a metal carrier usable for an exhaust gas purifying catalyst, by spirally winding a blank material into the form of metal carrier. The method according to the invention comprises the steps of: providing two to four sheets of flat metal foils having projections and holes over entire surfaces thereof, as the metal carrier blank material; spirally winding the flat metal foils under a mutually overlapped state, into a form of cylindrical body; and joining tip ends of the projections to corresponding surfaces of the flat metal foils.
In the present invention, two to four sheets of flat metal foils having projections and holes over entire surfaces thereof are spirally wound under a mutually overlapped state, into a form of cylindrical body; and tip ends of the projections are joined to surfaces of the flat metal foils; to thereby manufacture a metal carrier usable for an exhaust gas purifying catalyst.
The thus manufactured carrier of exhaust gas purifying catalyst defines an exhaust gas passage between flat metal foils which are neighboring in a radial direction with a spacing therebetween being defined by the projections. As such, the exhaust gas passage has a cross sectional shape resembling a rectangle, and the height of the projections can be arbitrarily selected, so that it is also possible to render the rectangle to be one having an aspect ratio of 4 or more by which there can be expected the fastest temperature rise as described above. Thus, there can be manufactured a carrier of exhaust gas purifying catalyst capable of effectively promoting temperature rise of catalyst after starting of engine.
Further, the carrier of exhaust gas purifying catalyst manufactured in the manner of the first invention defines the exhaust gas passage as described above, and such that the projections only to define the spacing between flat metal foils, so that the protruding angle of projection relative to the flat metal foil can be arbitrarily settled,
resulting in that it becomes possible to readily avoid occurrence of surfaces intersecting with an inner surface of the exhaust gas passage at an acute angle, thereby exiling occurrence of a problem that an unnecessarily large amount of wash coat liquid is coated onto a particular portion due to a surface tension, at the time of application of wash coat liquid onto the inner surface of the exhaust gas passage.
In this way, it becomes also possible to avoid cost increase due to adhesion of an excessive amount of wash coat liquid, and to avoid such a problem that deterioration of catalyst reaction efficiency due to reduction of catalyst carrying surface area leads to a long exhaust gas purifying catalyst.
In addition, there is not caused increase of heat capacity due to adhesion of excessive amount of wash coat liquid, so that there is not required a prolonged period of time for temperature rise up to the activating temperature of exhaust gas purifying catalyst after starting of engine. Further, there can be also avoided such a problem that the temperature rise of wall surface of exhaust gas passage just after starting of engine is delayed so that unpurified harmful substance continues to outflow.
In the carrier of exhaust gas purifying catalyst as manufactured in the first invention, the holes exist over entire surfaces of the flat metal foils, thereby enabling diffusion of exhaust gas in a radial direction from a carrier center portion where a flow rate of exhaust gas becomes large toward a carrier peripheral portion where a flow rate of exhaust gas becomes small. Thus, at the carrier peripheral portion where the temperature rise tends to delay, there is promoted the temperature rise to thereby enhance an exhaust gas purifying efficiency.
Advantageously, the tip ends of the projections of the flat metal foils are bent to extend along surfaces of the flat metal foils to which the tip ends of the projections are joined, and the each projections are joined to the each flat metal foils at the bent tip ends.
In this case, the joining areas between the projections and the flat metal foil are widened, thereby allowing enhancement of the joining strength between flat metal foils.
Advantageously, the width of one flat metal foil of the two to four sheets of flat metal foils is rendered to be identical with an axial length of the metal carrier after completion, widths of the remaining flat metal foils are rendered to be shorter by a range between 5 mm and 40 mm at the exhaust gas inflow side portion than the axial length of the metal carrier after completion, and the exhaust gas inflow side portion of the one sheet of flat metal foil in the above range is rendered to be a flat shape where no projections exist, a wavy metal foil having a width just for compensating the exhaust gas inflow side portions of the remaining flat metal foils, is overlapped with the exhaust gas inflow side portion of the one sheet of flat metal foil, and the spirally winding is performed under this state, and under this wound state, both sides of top portions of the wavy metal foil are joined to corresponding surfaces of the one sheet of flat metal foil, to thereby manufacture a metal carrier of an exhaust gas purifying catalyst.
In this case, the radially opposing portions of the flat metal foils forming the carrier of exhaust gas purifying catalyst are interconnected with each other at the exhaust gas inflow side portion by the wavy foils 20, instead of the aforementioned projections, so that the strength of the catalyst carrier at the exhaust gas inflow side portion can be enhanced.
Advantageously, the holes are formed by stamping the flat metal foils, and simultaneously therewith, those stamped and unsheared pieces protruded from the flat metal foils by the stamping are rendered to be the projections.
In this case, the holes are automatically formed upon forming the projections, resulting in unnecessity of providing another process for opening holes even if the holes are to be penetratingly formed for the aforementioned object, to thereby restrict cost increase, and weight increase is not introduced even when the projections are provided since the projections are parts of the material of the flat metal foil.
Advantageously, the stamping is performed by a punch after fixing the flat metal foil between a pressing plate and the die.
In this case, the formation according to the fourth invention can be performed in a manner that no wrinkles are caused in the flat metal foil, to thereby improve the quality of the catalyst carrier.
Advantageously, the projections are formed such that the merging portions between the projections and the flat metal foils are directed in a direction parallel to an axis of the metal carrier after completion or inclined within an inclination angle of 30xc2x0.
In this case, the projections do not give a resistance so large as to be problematic to an exhaust gas flow, nor cause an inconvenience for a spiral winding operation of the flat metal foils.
Advantageously, the projections are formed such that the merging portions between the projections and the flat metal foils are curved at a radius of curvature which is 0.7 to 50 times a length of each of the merging portions.
In this case, the projections do not cause a spring back when the projections are formed by stamping, thereby avoiding a problem that the spacing between flat metal foils deviates from what is predetermined, due to the spring back.
Advantageously, those paired projections and holes in the same raw aligning in a winding direction of the flat metal foils are formed such that the positions of merging portions between the projections and the flat metal foils are set at mutually farther hole side positions of the holes.
In this case, shearing stresses acting on the punch for stamping the flat metal foil are compensated with each other, when the projections are formed by stamping, resulting in that the lateral forces acting on the entire mold can be fully nullified.
Advantageously, the projections and holes are formed such that arranging patterns of the projections and holes are different from each other between neighboring flat metal foils, to thereby avoid such a situation that the projections align with the holes when the flat metal foils are overlapped with each other, thereby solving such a problem that the projections fail to maintain the spacing between the flat metal foils in a predetermined manner.